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Ceramic is a rigid material consisting of an infinite threedimensional network of sintered non-oriented inorganic, non-metallic crystalline grains, which are generally nontransparent. If the inner light scatter is eliminated, ceramics can become transparent or optical ceramics [1, 2]. With great potentials in optical windows, sensors and lasers, optical ceramics have received great attentions since 1950s [1]. As the gain medium of solid-state lasers, optical ceramics can combine the advantages of large size, high transparency, high mechanical and thermal stabilities, and ease for dispersing the luminous atoms and groups [3]. For comparison, single-crystal gain mediums can suffer high power density but are difficult to grow into large size, while glasses and polymers have reversed characteristics.

To make ceramics transparent, the inner pore and impurity should be minimized to zero, which requires highly pure, nanosized, and very uniform crystallite precursors as well as complicated fusing technology (Fig. 1)[4, 5]. The cubic crystal symmetries (absence of the birefringence which causes extremely high scattering losses) are generally required for the crystallites of optical ceramics, meaning that only a few inorganic materials can be used. Besides, ceramics sintering usually operates above 1000oC, which excludes organic and organic-inorganic hybrid materials [1].